1. Field of the Invention
The present invention relates to an image heating apparatus configured to heat an image formed on a recording medium by a heating belt, and to an image forming apparatus using the same.
2. Description of the Related Art
Regarding an image forming apparatus, it is generally known to transfer a toner image formed by an image forming portion that carries out an appropriate image forming process such as an electrophotographic process, an electrostatic recording process, a magnetic recording process and others to a recording medium by an intermediate transfer method or a direct transfer method, and to fix the non-fixed toner image transferred to the recording medium onto the recording medium by heating and pressing by a fixing apparatus, i.e., an image heating apparatus. It is also known to control the glossiness of an image by heating the image fixed to the recording medium again by the fixing apparatus.
Japanese Patent Application Laid-open No. S63-313182 for example discloses a belt-type heating apparatus, as such fixing apparatus, that allows a width of a nip portion through which a recording medium is passed to heat an image to be widened and a warm-up time to be shorted. That is, conventionally, there has been known a configuration of fixing an image on a recording medium by forming a nip portion by a heated belt as a fixing member and a pressure roller and by passing the recording medium on which a toner image has been formed through this nip portion.
However, because the recording medium draws heat from the fixing member in passing through the nip portion, Japanese Patent Application Laid-open No. 2004-37555 discloses a configuration of heating the fixing member not only from an inner side but also from an outer side thereof. This configuration enables shortening of the time required to attain a required surface temperature of the fixing member because the fixing member is heated also from the outer side.
By the way, an image forming apparatus capable of executing a series of image forming jobs while properly identifying and using different types of recording media is put into practical use these days. This type of image forming apparatus makes it possible to use thick sheets as front and back covers and to use thin sheets as a body portion interposed between those covers. Such image forming apparatus also makes it possible to scan a plurality of groups of originals by a scanner at once, and then to copy a first scanned group by thin sheets and to copy a second scanned group by thick sheets for example. In the same manner, the image forming apparatus can readily realize such technique of printing while changing over types of sheets per each of plurality of image datum supplied from a client apparatus such as a personal computer.
It is preferably to change the quantity of heat applied by the fixing apparatus in accordance with the types of recording media in fixing images when properly identifying and using such different types of recording media in forming images. For instance, it is preferably to increase the quantity of heat to be applied in fixing an image on a recording medium, such as a thick sheet whose basis weight is large and to decrease the quantity of heat to be applied in fixing an image on a recording medium, such as a thin sheet, whose basis weight is small. Therefore, it is preferably to change over preset temperatures of heating members provided inside or outside of the fixing member for heating the fixing member in accordance to the types of recording media. However, it takes time to increase/decrease the temperature of the heating member to a desirable temperature just by changing over the preset temperatures, thus, decreasing the productivity of the apparatus.